1. Field of the Invention
The present invention relates to an optical element, optical system and optical apparatus that have an antireflection coating.
2. Description of the Related Art
A dielectric multilayer film, generally called a multi-coated film, is widely used as an antireflection coating. This is configured to adjust the amplitudes and phases of reflected waves generated on a surface and an interface of each coating by laminating thin films with different refractive indexes and to reduce reflected light by making the reflected light fluxes interfere each other. Japanese Patent Laid-Open No. (“JP”) 2010-271404 discloses a lens that has an antireflection coating configured with five layers of dielectric thin films, which are formed on a substrate with a refractive index of 1.90 or more using the vacuum evaporation deposition or electron beam evaporation method.
Each of JPs 2008-233880 and 2009-42472 discloses a lens that has a fine, antireflection structure shorter than a used wavelength. The light that has entered the structure shorter than the wavelength is configured to behave as if the light entered a medium having a refractive index (effective refractive index neff) equivalent to a volume ratio of a material of the fine structure. Then, the following equation is established based upon a Lorentz-Lorenz equation with a refractive index nm of the material of the fine structure and space filling factor ff:(neff2−1)/(neff2+2)=ff(nm2−1)/(nm2+2)  (1)
JP 2008-233880 discloses an antireflection coating in which an effective refractive index continuously changes from a light incident side to a substrate and a plate crystal containing aluminum oxide as a primary ingredient is used for the top layer. Since the light reflection takes place in the interface between two substances with different refractive indexes, the reflected wave is suppressed in the structure having such a continuously changing refractive index. JP 2009-42472 discloses an antireflection coating that has a top layer as a low-refractive-index layer having a fine undulation structure and a multilayer film containing a two or more layers between the top layer and the substrate. The multilayer film disposed between the top layer and the substrate has a gradually decreasing refractive index from the substrate side to the fine undulation structure.
Since the antireflection coatings disclosed in Japanese Patent Laid-Open Nos. 2010-271404 and 2009-42472 adjust the amplitude and phase of a reflected wave generated on the surface or interface of each film, the antireflection characteristic deteriorates for obliquely incident light (in particular, with an incident angle of 30° or more) in which the interference condition is not satisfied. In addition, because of the high thickness sensitivity, the antireflection performance remarkably deteriorates when the thickness varies by only several percentages. The antireflection characteristic of the antireflection coating disclosed in JP 2008-233880 deteriorates for a substrate having a high refractive index because the refractive index difference is high in the interface between the fine undulation structure and the organic resin layer.